The vast majority of arterial hypertension is “essential hypertension,” meaning no identifiable cause. I assume this is (misleadingly) saying that of the secondary (non-essential) hypertension cases, the condition that this scan detects is the most common.

From TFA:

" The scan found that in two thirds of patients with elevated aldosterone secretion, this is coming from a benign nodule in just one of the adrenal glands, which can then be safely removed."

So it seems that 2/3 of the cause of HBP is elevated aldosterone, but 1/20 of cases this can be fixed with a not-so-invasive procedure.

"Most common cause", singular.